In an IP or VoIP network using cable routers and hybrid fiber coaxial (HFC) access networks, a cable interface such as a cable modem termination system (CMTS) is necessary. FIG. 1 shows a diagram of communication system 10, comprising the IP network 70, system headend 75, including primary routers 21, backup router 22 and cable modem termination system (CMTS) 20; HFC access network 40, PSTN 15 and end users 25, 26. Detection and status reporting of CMTS cable connections can prevent a switchover from a primary router 21 to a backup or redundant router 22 from occurring under network failures caused by external cable problems, such as improper connection, removed cable connection to one or more of its RF ports, cable break, etc. Switching to backup equipment during external faults does not solve the problem, and effectively ties up both the primary and backup equipment needlessly. This reduces the overall reliability of the system. It is also desirable to have the ability to quickly detect and locate a fault in the HFC cable network to allow prompt repair and system recovery.
Prior art solutions include injection of a signal across the center conductor or coaxial shield at the source of a cable connection. The presence of the known signal is then detected by either a dedicated detector placed at one or more points along the signal path or through detection of the radiated signal through some type of inductive coupling. Connection of specific continuity detectors to cable or near the cable in the HFC network is a not viable option where a CMTS designer has little control over the externally coupled cable network. In addition, even in cable plants where it would be possible to connect external cable monitoring equipment, the CMTS's need for such equipment and its maintenance may be unfavorable to network managers.
An alternative solution is time domain reflectometry (TDR) which can sense cable discontinuities when the cable length is great enough. However, implementing TDR in the CMTS or matrix switch is prohibitive with respect to cost, size and complexity. TDR is also an intrusive test and is ineffective on short cable lengths.